How to Bleed Air from a Hydraulic System

[MikePhua]

Air trapped in a hydraulic system can significantly impair the system’s performance. It causes erratic operation, increased wear, reduced lifting capacity, and can even lead to system failure if not addressed properly. Bleeding air from a hydraulic system is a crucial maintenance task that ensures smooth and efficient operation. This article will provide a comprehensive guide to understanding why air can get trapped in a hydraulic system, how to effectively bleed it out, and the steps involved in the process.
Understanding Hydraulic Systems and the Role of Air
Hydraulic systems are designed to use pressurized fluid to perform tasks such as lifting, pushing, or rotating. These systems are highly efficient and reliable, but like any mechanical system, they can experience issues that affect their performance. One of the common problems is the presence of air in the hydraulic fluid.
Air typically enters the system in one of the following ways:

• Improperly filled hydraulic reservoirs: If the fluid level is too low or air enters during fluid changes, it can create air pockets.
  
• Leaks in the system: Any break in the hydraulic lines, seals, or fittings allows air to enter the system.
  
• Fluid cavitation: When hydraulic fluid is subjected to rapid pressure changes, it can form bubbles, which later collapse, releasing air into the system.
  
• New installations or maintenance: During system repair or installation, air can unintentionally be introduced.
  

When air is present in a hydraulic system, it behaves differently from hydraulic fluid. Air compresses and expands, preventing the hydraulic system from operating with the desired precision. This leads to slow response times, a "spongy" feel in controls, or a lack of force during operations.
The Importance of Bleeding Air from the System
If air is not removed from the hydraulic system, the system's overall performance will deteriorate. The effects of trapped air include:

1. Erratic Operation: The hydraulic system will respond unpredictably, leading to jerky or inconsistent movements.
   
2. Reduced System Efficiency: Air-filled systems are less efficient at transmitting power, reducing the effectiveness of lifting, digging, or other hydraulic operations.
   
3. Increased Wear: Air in the system causes increased friction between moving components, accelerating wear on parts like pumps, cylinders, and seals.
   
4. Overheating: Trapped air can lead to excessive heat buildup, as the system struggles to operate under inefficient conditions.
   
5. Pump Damage: Prolonged exposure to air can damage the hydraulic pump, leading to costly repairs or replacements.
   

How to Bleed Air from a Hydraulic System
There are several methods to bleed air from a hydraulic system. The right method will depend on the type of system you have and the tools at your disposal. Here are the general steps to follow:
1. Safety First
Before starting any work on your hydraulic system, ensure that the equipment is turned off and properly secured. Engage the parking brake, and wear appropriate safety gear such as gloves and goggles. Hydraulic systems are under high pressure, so working on them without proper precautions can be dangerous.
2. Check Fluid Levels
Begin by checking the fluid levels in the hydraulic reservoir. Low fluid levels are often the root cause of air entry. Ensure that the reservoir is filled to the manufacturer’s recommended level. If there is a low level, add the correct type of hydraulic fluid to the system.
3. Locate the Bleed Valve
Many hydraulic systems are equipped with a bleed valve or a vent plug to allow for the release of air. This valve is typically located near the hydraulic pump or at high points in the hydraulic system where air tends to accumulate. If no specific valve is available, you may need to loosen hydraulic fittings to allow air to escape.
4. Activate the Hydraulic System
To begin the bleeding process, activate the hydraulic system to force the trapped air out. This is usually done by moving the hydraulic levers or engaging the machine's hydraulic functions (like raising the boom or lowering the bucket) to force fluid through the system. Be sure to operate all hydraulic movements, as air can get trapped in various parts of the system.
5. Open the Bleed Valve
Once the system is engaged, open the bleed valve slowly. This will allow the trapped air to escape from the system. It’s essential to open the valve gradually to avoid sudden fluid bursts, which could cause fluid loss or damage to the system. Allow the air to escape until only fluid is coming out.
6. Close the Bleed Valve
Once you notice that only hydraulic fluid is coming out of the valve, close the bleed valve securely. Tighten the valve to ensure there are no leaks.
7. Test the System
After bleeding the system, test the hydraulic system’s operation by using all the hydraulic functions. If the air has been successfully removed, you should notice smoother operation and more precise control. If the system still feels sluggish or erratic, you may need to repeat the bleeding process or check for additional air leaks.
Additional Tips for Effective Air Bleeding

1. Use the Correct Hydraulic Fluid: Always use the hydraulic fluid recommended by the manufacturer for your equipment. Incorrect fluid can lead to contamination or poor performance, making it harder to bleed air from the system.
   
2. Check for Leaks: If the air keeps returning, it’s likely due to a leak in the system. Inspect hydraulic hoses, fittings, and seals for any signs of wear or damage. Tighten or replace any faulty components to prevent air from re-entering the system.
   
3. Bleed Multiple Times: In some cases, air may not be entirely removed on the first attempt. Bleeding the system multiple times ensures all trapped air is eliminated.
   
4. Monitor the System: After bleeding the system, continue to monitor it during operation. If you notice that air is re-entering, it could be an indication of a serious issue, such as a pump failure or seal damage.
   

Preventing Air From Entering the Hydraulic System
While bleeding air from the system is an important part of hydraulic maintenance, it's also critical to prevent air from entering the system in the first place. Here are a few tips to help:

• Ensure Proper Maintenance: Regularly inspect and maintain your hydraulic system. Clean filters, check hoses for wear, and ensure the system is sealed correctly.
  
• Avoid Low Fluid Levels: Always maintain the proper fluid levels. Hydraulic fluid should be topped off as needed to prevent air from being drawn into the pump.
  
• Avoid Sudden Movements: When using hydraulic machinery, try to avoid sudden or excessive movements, as this can create cavitation and cause air to enter the system.
  

Conclusion
Bleeding air from a hydraulic system is a necessary maintenance task that should not be overlooked. Air trapped in the system can lead to reduced efficiency, increased wear, and potential equipment damage. By following the proper steps, you can ensure that your hydraulic system operates smoothly, keeping your equipment in optimal condition for longer periods. Regular maintenance, careful monitoring, and swift action in case of air buildup will help maintain the health of your hydraulic systems, ensuring they perform at their best.